A. Field of the Invention
The present invention relates to a method for the production of a homogeneous steel, more particularly to a method for the production of a steel having excellent resistance to cracking.
B. Description of the Prior Art
In general, segregation of alloy elements and impurity elements takes place within steel castings particularly within those produced by continuous casting (referred to as "CC" hereinafter) as well as other steel ingots in the course of solidification. Throughout specification, the term "casting" is used to include steel castings produced by CC and other "steel ingots" produced by the conventional steel casting methods. Particularly, in the CC casting, a segregation zone called the "central segregation zone" occurs in the central part of the casting in the direction of its thickness. Unless this segregation is eliminated or reduced in a subsequent step, it remains in the casting and causes various types of cracks. For instance, cracks tend to occur in the heat affected zone (HAZ), when a heavy plate obtained from a casting is welded even though careful attention was given to crack prevention in the design of the steel composition and the manufacturing processes. This is because the condition of the alloy in the central segregation zone is considerably different from the designed steel composition because of segregation of alloy elements and impurity elements.
The best way to prevent the segregation of alloy and impurity elements in a casting is to prevent it at the time of occurrence, namely, in the course of solidification after casting. However, with the present available steel casting techniques, it is difficult to prevent the segregation of alloy and impurity elements completely. Therefore, it is necessary to eliminate or reduce the segregation occurring in castings in subsequent working steps. In order to eliminate or reduce segregation caused in casting, there has been used a homogenizing treatment. This treatment comprises the step of heating the casting uniformly a temperature of 1250.degree.-1300.degree. C. so as to diffuse the segregated elements, thereby to eliminate or reduce the segregation.
However, if homogenizing treatment is to be used to adequately reduce segregation so as to obtain a steel having excellent resistance to cracks, it is necessary to considerably prolong the heat treatment. In view of this, a compromise is made from the viewpoint of productivity or production cost and a more practical heat treatment of around 10 hours is used.
Accordingly, adequate reduction of segregation is never achieved by the conventional method. Furthermore, since the temperature of the homogenizing treatment is high, in the range of 1200.degree.-1300.degree. C., it is disadvantageous from the viewpoint of production cost and energy-saving.
Steel stock has conventionally been produced by the blooming or slabbing of castings. The purpose of blooming or slabbing in this case is to adjust the size of the casting to obtain a steel stock which the rolling mill is capable of rolling into a product of predetermined size with good yield. Quite recently, there has been a tendency to reduce the heating and rolling temperatures in blooming or slabbing but the purpose of this is to save energy and not to reduce the segregation within the casting. On the contrary, it is usually considered that the lowering of both the heating and rolling temperatures in the blooming step has an adverse effect on the reduction of segregation. Besides, the step of reheating the casting after blooming is directed to heating it to the temperature required for subsequent rolling.
Quite recently, with a view to preventing the failure of line pipe steel caused by the penetration of hydrogen as a consequence of corrosion, a method for maintaining a CC casting at a temperature in the range of 1150.degree.-1300.degree. C. for a prolonged period of time has been proposed. Also, another method for obtaining a steel with a high toughness has been also proposed, in which the casting is rolled at a reduction ratio of more than 10%, then cooled to less than 750.degree. C. on the average, and subsequently held at a temperature in the range of 750.degree.-500.degree. C. for more than 10 minutes in order to precipitate fine AlN. Neither of these methods is an effective means for eliminating the segregation of alloy or impurity elements in the steel.